1. Field of the Invention
The present invention relates to a circuit board and, more particularly, to a light-emitting diode (LED) circuit board capable of generating uniform luminance.
2. Description of the Related Art
Conventionally, when LEDs are applied to lighting and backlight applications, the LEDs are usually mounted on a substrate. All traces on the substrate take the form of a single trace layout. Luminance of each LED is determined by current flowing through the LED. The most common way of connecting the LEDs is done by either series connection or parallel connection. However, in the event of the series-connected LEDs, driving voltage increases with the number of the LEDs. When the driving voltage is insufficient to drive the LEDs, additional voltage-boosting circuit or LED power driver is required to provide higher voltage to normally drive the series-connected LEDs and certainly increases the production cost.
With reference to FIG. 5A, after being connected in parallel, multiple LED strings 70 are connected to a power source 80. Thus, all the LED strings receive a same voltage, which is the same as voltage supplied by the power source 80. For example, each LED string 70 includes an LED 71 receiving power with a voltage value identical to that provided by the power source. As traces connecting the LEDs 71 and the power source 80 have their resistance values, the voltage received by each LED 71 depends upon a trace length between the LED 71 and the power source 80. The longer the trace length between the LED 71 and the power source 80, the more the resistance value of a circuit loop formed by traces between the LED and the power source. Although the power source 80 supplies power with a constant voltage value, current flowing through different LEDs 71 and varying with the circuit loops between the respective LEDs 71 and the power source 80 causes different or inconsistent luminance of the LEDs 71.
With reference to FIG. 5B, each LED string 70 further includes a resistor R. For example, each LED string 70 has a resistor R connected to an LED 71 in series. The resistance value of the circuit loop between each LED string and the power source 80 can be adjusted by the resistor R to lower the difference in resistance values of the circuit loops between the respective LED strings 70 and the power source 80 and suppress current flowing through each LED 71. However, more current loss arises from the use of the resistor R in such an approach. Hence, voltage of the power source 80 needs to be boosted to increase voltage of each LED string 70, such that same current flowing through the LEDs 71 can be maintained. As each LED string 70 requires a series-connected resistor R, a total number of the resistors R increases with the number of the LED strings 70 to cause manufacturing cost increase. Additionally, the power utilization efficiency is dropped because voltage-boosting operation performed on the resistors R.
With reference to FIG. 6A, each LED string 70 includes multiple LEDs connected in series. The power source 80 is further connected to an LED power driver 81. The LED power driver 81 boosts voltage of the power source 80 and supplies constant current to each LED string 70. Although the LED power driver 81 can supply constant current to each LED string 70, the resistance value of each circuit loop between each LED string 70 and the power source 80 is still determined by a trace length of the circuit loop. Similarly, the issue of inconsistent luminance among different LED strings 70 also happens because of different current values through the LED strings 70 as a result of differences of the resistance values of different circuit loops.
With reference to FIG. 6B, another LED power driver 81 is employed. The LED power driver 81 includes multiple channels electrically connected to multiple LED strings 70 respectively. By using the LED power driver 81 to adjust current flowing through each LED string 70, identical current values across the LED strings 70 can be ensured to tackle the issue of inconsistent luminance arising from different current values across the LED strings 70. As the LED power driver 81 needs to receive information about current flowing through each LED string 70 for adjustment of current flowing through the LED string 70, when the LED strings 70 increases in number, the LED power driver 81 requires more channels in connection with the LED strings 70 and a demand for a more high-end and expensive LED power driver 81 becomes necessary and inevitably increases the overall cost.
Moreover, a conventional parallel circuit for tablet with enhanced power utilization efficiency includes a first constant voltage layer and a second constant voltage layer formed on a top surface and a bottom surface of a substrate respectively. The first and second constant voltage layers are connected to a power supply respectively through two power connection points. The first constant voltage layer has at least one insulating zone. Each insulating zone has a light-emitting unit formed therein. One electrode of the light-emitting unit is connected to the first constant voltage layer, and the other electrode thereof is connected to the second constant voltage layer through a conducting trace. When the power supply outputs a low voltage to the first constant voltage layer, resistance values everywhere on the first constant voltage layer are identical. However, due to the conducting trace required by the light-emitting unit to electrically connect to the second constant voltage layer through an opening formed through the first substrate, the production process and composition of the parallel circuit are complicated. Besides, the first constant voltage layer and the second constant voltage layer are fully spread over the circuit board to result in a higher material cost. Meanwhile, the first constant voltage layer and the second constant voltage demand for larger areas and in turn consume more power.